An optical spectroscopic HR diagram for low‐mass stars and brown dwarfs in Orion
An optical spectroscopic HR diagram for low‐mass stars and brown dwarfs in Orion
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猎户座低质量恒星和褐矮星的光学光谱 HR 图
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发表时间：
2007
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通讯作者：
U. Hertfordshire
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作者：
F. Riddick;P. Roche;P. University;U. Oxford;U. Hertfordshire
由于个别天体的年龄和光谱类型-温度标度不确定，适用于年龄只有几百万年的天体，因此在恒星形成区的年轻低质量恒星和褐矮星的质量和温度还没有很好的确定。利用多目标光谱学，猎户座梯形星团中的45颗低质量恒星和褐矮星已经被分类，其中44颗被确认为真正的星团成员。所获得的光谱类型已被转换为有效温度，使用的温度范围介于矮星和巨星之间，这是适合年轻的前主序天体。这些物体被放置在覆盖有理论等时线的Hertzsprung-Russell（HR）图上。低质量恒星和高质量的亚恒星天体被发现聚集在100万年等时线周围，而许多低质量的亚恒星天体则位于远高于这条等时线的位置。大多数天体的平均年龄为100万年。假设源的共演化性和平均年龄为100万年，物体的质量估计为0.018至0.44兆焦耳。该光谱还允许调查的表面重力的对象的测量钠偶极子等效宽度。除了一个可能的例外，所有天体都有低重力，与年轻的年龄一致，梯形天体的Na指数系统地低于Chamaeleon中的年轻恒星和棕矮星，这表明820 nm的Na指数可能提供一种敏感的方法来估计年轻星团的年龄。
The masses and temperatures of young low-mass stars and brown dwarfs in star-forming regions are not yet well established because of uncertainties in the age of individual objects and the spectral type-temperature scale appropriate for objects with ages of only a few Myr. Using multi-object optical spectroscopy, 45 low-mass stars and brown dwarfs in the Trapezium Cluster in Orion have been classified and 44 of these confirmed as bona fide cluster members. The spectral types obtained have been converted to effective temperatures using a temperature scale intermediate between those of dwarfs and giants, which is suitable for young pre-main-sequence objects. The objects have been placed on a Hertzsprung-Russell (HR) diagram overlaid with theoretical isochrones. The low-mass stars and the higher mass substellar objects are found to be clustered around the 1 Myr isochrone, while many of the lower mass substellar objects are located well above this isochrone. An average age of 1 Myr is found for the majority of the objects. Assuming coevality of the sources and an average age of 1 Myr, the masses of the objects have been estimated and range from 0.018 to 0.44 M⊙. The spectra also allow an investigation of the surface gravity of the objects by measurement of the sodium doublet equivalent width. With one possible exception, all objects have low gravities, in line with young ages, and the Na indices for the Trapezium objects lie systematically below those of young stars and brown dwarfs in Chamaeleon, suggesting that the 820 nm Na index may provide a sensitive means of estimating ages in young clusters.